Rick Riordan announced the title of the second book in the back of The Sword of Summer. The cover and an exclusive first look at chapter 1 were revealed on April 28th, 2016. 
Thor's Hammer is missing again. The Thunder God has a disturbing habit of misplacing his weapon- the mightiest force in the Nine Worlds- but this time the hammer isn't just lost. It has fallen into enemy hands. If Magnus Chase and his friends can't retrieve the hammer quickly, the mortal worlds will be defenseless against the onslaught of the Giants. Ragnarök will begin. The nine worlds will burn. Unfortunately, the only person who can broker a deal for the hammer's return is the gods' worst enemy- Loki, and the price he wants is very high.
"A Meeting Cut Short and Roof Chase"
Samirah al-Abbas meets Magnus Chase at The Thinking Cup after not seeing him for six weeks. After catching up a bit, Sam tells him they've got an "informant" to meet. Before she can elaborate, her phone rings to notify her of a "possible code 381: heroic death in progress" and she has to leave. Before she does, she tells Magnus to stay and wait for her "informant", who turns out to be Thor's goat Otis. Otis tells him that Thor's missing hammer Mjolnir is in a wight's barrow in Provincetown, and that something had been stalking him. He begins to issue a warning for Magnus to pass on to Samirah, but is cut off with an ax to the heart. Magnus chases the attacker, who was lurking on the roof opposite their side of the street, but misses a leap and narrowly avoids sliding off a church roof. The "goat assassin", who is standing at the top of the roof, tells Magnus to stay away from Provincetown. He flees and Magnus throws his enchanted sword Jack after him, which buys him time to pull himself up. He injures the assassin, but a howl in the distance causes him to freeze up. His opponent gives him one last warning before disappearing over the roof's edge. Magnus meets back up with Sam at the café, who now has a mysterious bruise under her left eye, and tells her about what happened. They both begin to suspect Loki is behind this and decide that they need to retrieve Thor's hammer as soon as possible in order to prevent a horde of Jotunn from invading Midgard. After Magnus tells Sam about the supposed location of the hammer, she says that they will require Blitzen and Hearthstone's help to successfully enter the wight's tomb. Sam reveals that they have been hiding in one of Mimir's safe houses, but does not tell him why. She promises to explain everything to him later and that she will see him at the evening feast in Valhalla, where she will also introduce him to her "brother", the einherji she picked up earlier.
“Training with a New Roommate”
Magnus returns to Valhalla and takes the elevator to floor nineteen, where he is immediately run over by a cheetah. Thomas Jefferson Jr., Halfborn Gunderson and Mallory Keen, who were in pursuit of the wildcat, reveal that it is actually their new hallmate, a shape-shifter who was not happy to be there. When they had gone to welcome him, they were met with "first-rate insults" and attempted murder. They lead Magnus to X's old room, which now belonged to the newcomer, and he sees that it has been completely trashed. He notices that the floor is littered with broken pieces of ceramic bowls and cups, and when he inspects one of them, he finds the initials "A.F." scratched onto it as well as a decorative stamp of two snakes curled in an "S" pattern with their tails looped around each other's heads. Halfborn tells him that the stamp is a symbol of Loki's.
Magnus returns to his suite and ponders on the conversation he last had with his cousin Annabeth, where he learned that she was the a daughter of Athena and that the Greek pantheon also existed in the world. He proceeds to inspect the photos on his shelf and inspects the one with his mother, his uncle Frederick, and his uncle Randolph and notices that a mark has appeared on Randolph's cheek that matches the stamp on the pot shard. Magnus summons Jack and questions him about the mark, which Jack also says is Loki's. Magnus and Jacks discuss the appearance of another child of Loki appearing just as the mark appears on Randolph's photo as well as Mjolnir being lost again. T.J. pops in to bring them to the daily battle, which, being a Thursday battle means in addition to the regular fights to the death, the einherji must also face lindworms. They meet up with Mallory and Halfborn Gunderson and are introduced to the new hall mate Alex Fierro, a gender fluid and transgender teenager as well as a child of Loki. The group is attacked by one of the lindworms, which Alex reveals is named Grimwolf. This is an unexpected situation as Grimwolf is one of the oldest lindworms and normally lives at the roots of Yggdrasil, which causes Alex to suspect that it was sent by Loki. As Magnus tries to distract Grimwolf for Alex, he experiences a searing pain on his face and he suddenly sees himself in Randolph's study with Loki standing over him. Loki talks to Magnus through Randolph and tells him that he is giving him an invitation to a wedding where Sam is the bride and that Magnus is tasked with acquiring the bride price. Loki claims that he will give Mjolnir in exchange for Sam's cooperation in the wedding, but when he takes notice of Alex he tells Magnus she is not invited to the wedding and as his vision turns back to Valhalla, Magnus' body begins to overheat. Alex explains that Loki is channeling his power through Magnus and that the explosion will destroy most of the courtyard, so Alex decapitates Magnus to cut him off from Loki.
While dead, Magnus dreams about Randolph being on a yacht in the middle of a storm with his family (his wife Caroline, his oldest daughter Aubrey, and his youngest daughter Emma) and witnesses them be thrown into the ocean by a wave. He then has another dream where his head is floating in a bathtub and he sees Mimir talking to Blitz and Hearth about having to go somewhere. While Hearth seems concerned about Blitz's safety, Blitz insists that they have to go and help Magnus. Magnus goes into another dream where his head is inside a pickle jar inside some kind of bar, where he overhears the giants Thrym and Thrynga talking about Thrym's marriage to Sam. After being resurrected, Magnus makes his way to evening banquet just in time to witness Alex's heroic deed. Its revealed that Alex tried to defend an elder man (possibly a demigod) from three wolves who were children of Fenris, but was eventually overpowered and killed by the wolves. Alex's deeds are met with great approval from the einherji and the thanes. After the banquet Magnus meets with Sam and discusses his dreams with her. She explains that Loki visited her grandparents and revealed everything to them before dropping off an invitation.
"An Undead Trap"
The next day, the two of them fly on a winged horse to Cape Cod to meet Blitz and Hearth. When they meet up, a prophecy is revealed to have been read by Hearth which announces Blitz's death. They find the wight's barrow in an ancient underground tomb. They open a coffin with the words "Blood River" written all over it, causing the wight inside to spring to life, which introduces itself as Gellir, who is in possession of a sword called Skofnung. He also reveals that Mjolnir is not in the barrow. The wight says he needs to kill the group for him and his men to break free of the tomb but they need to hold a meeting before they get to it. Evetually they agree to kill the four of them, but they manage to defeat the wight and his men. Soon after, however, Randolph and Loki appear. Loki causes Sam to stop breathing and orders Randolph to unsheathe Skofnung, despite the rule about not being drawn in the presence of women. Another rule is that it can't sheathed until it has drawn blood, so Loki has Randolph stab Blitz with the sword. Magnus tries to heal the wound but Loki says the only way to heal the wound is to used the Skofnung Stone. To save Blitz form bleeding out Magnus collapses the barrow and turns Blitz to stone.
"Paying off Hearthstone's debt"They go to the Provincetown airport where Sam's instructor, Barry, allows them to borrow his plane to fly to Alfheim. Amir shows up at the airport and joins them on the flight. Before taking off Barry is possessed by the goat killer who yells at them for falling into Loki's trap and then tells them to seek out Heimdall to find him in the Jotunheim and to bring Alex along. Hearth tells Magnus that in order to reach Alfheim, they have to jump out of the plane. After reaching Alfheim, they are approached by two police elves named Sunspot and Wildflower, who escort them to Hearth's home. They are greeted by a hulder named Inge, who serves Hearth's household and Magnus is then introduced to Hearth's father, Alderman. He reveals that Hearth's mother, Greta, was involved in a car accident that resulted in her death and that he blames it on Hearth running away. Magnus finds that the Skofnung Stone is in Mr. Alderman's possession and must be given willingly to be able to be used. They ask to borrow the stone, but Mr. Alderman says that Magnus, as a son of Frey, needs to help Alderman raise his social standing and Hearth must pay his wergild. It is revealed that Hearth's brother Andiron was killed by a monster in a well behind the house and Hearth was blamed for his death because he couldn't hear his brother being attacked. As a result, Hearth must pay a wergild, a blood debt in which he must accumulate enough gold to completely cover the skin rug of the beast. Inge mentions that there is a dwarf in Alfheim called Andvari, or the Careful One, who has a horde of gold large enough to cover the rug. She tells them he is living as a fish in the river in the woods behind the mansion. Magnus and Hearth manage to catch Andvari and get him to reveal the location of his horde. They also take Andvari's cursed ring, even though he warns them that all who have taken it suffer terrible consequences. They use Andvari's treasure to pay the wergild and use the Skofnung Stone to heal Blitz after turning him back to normal in running water. Later that night, Mr. Alderman throws a party to be seen with Magnus, but the curse of the ring causes him to go crazy and start accusing the guests of trying to steal his treasure. He summons water spirits called Nøkks as security and makes them start attacking the guests. The three of them manage to escape the mansion, Jack cuts a portal between worlds near the well, and they wind up in Midgard.
Magnus leaves Blitz and Hearth to recover and goes to the roof of the building where Sam, Alex, Halfborn Gunderson and Amir are waiting near the Boston Citgo sign. They are invited by Heimdall to walk upon the Bifrost and talk to him. Heimdall tells them that a mass of giants are discussing an alliance with Loki through the wedding of Thrym and Sam before their invasion of Midgard takes place. Sam makes a vow before Heimdall, as he is the guardian of the sacred marriage bed, that she will never marry or pretend to marry anyone other than Amir. While looking for the goat killer, Heimdall takes a picture of a place in the Jotunheim called Utgard Lanes that has a sign telling Magnus to come and get the information to interfere with Loki's plan, and Magnus realizes the goat killer was actually Utgard-Loki.
The next day before leaving for Jotunheim, Magnus and Alex sneak into Randolph's office for information. They find an invitation behind a photo of Bridal Veil Falls in New Hampshire and suspect it might be the location for the wedding. They regroup with the others and are joined by Stanley. Sam flies with Hearth to the Jotunheim, while Magnus, Alex and Blitz ride Stanley. Soon after entering they are knocked off Stanley by a low hanging branch, which turns out to actually be the shoestring of a giant named Tiny. Tiny points them in the direction of Utgard Lanes, but in order to have him vouch for them he tells them they need to bring his giant bowling ball bag to the Lanes by tomorrow morning. Blitz gets the idea to give the bag a name to make it a dwarven item and enchant it to magically change size, using Jack as a needle to sew in magical thread made with Magnus' hair and Alex's blood. As Blitz and Jack work on the bag, Magnus and Alex get to talking and he learns that Loki is Alex's mother. After the bag is finished, they travel to Utgard Lanes and meet up with Utgard-Loki who reveals to already have Sam and Hearth. Utgard-Loki explains that even though he wants to prevent Loki's plan, the five of them must prove themselves to the other giants by performing impressive feats. The first includes an ax-throwing contest between Sam and a kid giant called Little Billy. Little Billy seems to win, but Sam throws her ax into him and reveals him to be Fear, which can only be defeated by being attacked head-on. Next, Hearth challenges a giant to arcade games and defeats him by surpassing the giant's skee-ball score with his pinball score. Blitz's challenge is a complete makeover where he makes an extremely ugly giant attractive (at least to other giants). Magnus and Alex take on the giants in bowling. At first they are unable to hit the pins because there is a portal to Midgard in front of their pins, but they manage to close it and beat the giants. With that they buy five minutes for Utgard-Loki to give them information before the giants are allowed to chase them down. Utgard-Loki reveals that Loki intends to free himself with the Skofnung Sword and manipulated the events of Mjolnir's theft to get the sword, since having the sword as the bride price means that it will go to the father of the bride by jotun tradition.
As the five of them are running from the giants, Hearth directs them to a rowan tree nearby, which suddenly reveals a portal that they all jump through. They find themselves summoned by the goddess Sif to her and Thor's palace, Bilskirnir, in Asgard. Sif reveals that she was watching the group, waiting to summon them when they got the information they needed to retrieve Thor's hammer. Sif gives Hearth wooden runes, but warns him that he will one day have to acquire the 'inheritance' rune to fully unlock his potential. She then directs them to Thor's 'man-cave' to discuss their plan to retrieve Mjolnir with him. They explain that Mjølnir is to be used as the morning after gift in the earth giant Thrym's marriage to Sam and in the meantime is being hidden under eight miles of earth. Thor says that the bride has the right to insist that the gift be summoned in order to bless the ceremony and ensure that the hammer can be obtained. Alex decides to pose as Sam so that she can keep her vow and because he/she believes that he/she can resist Loki's control over his children. After they agree to infiltrate the wedding with Alex posing as Sam, Thor gives them his chariot to ride to the wedding. Thor explains he can concentrate to see through Otis and Marvin's eyes and that when the hammer is summoned by Thrym, he will lead a squad of gods and einherjar underground to retrieve the hammer and slaughter the giants. He then gives Blitz and Hearth "Thor's Hall Pass" in order to allow them to enter Valhalla and alert the einherjar to the plan.
Magnus, Sam and Alex get dressed up for the wedding and read the invitation to find that the entrance to Thrym's lair is at Bridal Veil Falls for five minutes. They are let into the lair by Thrynga. After participating in the wedding feast, the giants reveal that the location for the actual ceremony is in Loki's cavern. They arrive to Loki chained up on the floor with a large snake dripping venom on his face, with his wife Sigyn at his side, trying to keep the venom out of his eyes with a small bowl. Loki asks Alex to come forward to confirm that the bride is Sam. Alex transforms to look like Sam, and Loki assumes Sam is Alex and the ceremony gets underway. Loki has Randolph retrieve the Skofnung Sword from Magnus and just as he tells him to cut him free, Alex breaks character and attacks Thrym. Magnus is surrounded by earth giants and Loki frees Sam as he orders Randolph to draw the sword, despite its rule on women being present, and to cut him free before the blade disappears from breaking the rule. Before Loki is completely cut free, the cavern if broken into by Hearth, Blitz and the einherjar of floor nineteen. Despite the reinforcements arriving and the gods on their way, Randolph manages to sever Loki's final bond, freeing him. Loki's first action is to kill the snake that had been dripping venom on him. Before he engages any of the other warriors, the gods managed to find their way to the cavern, so Loki opens a hole in the ground to retreat and falls in along with Sigyn. Randolph falls in as well, despite Magnus trying to pull him back up. The gods finally manage to break into the cavern, Thor leading the charge, followed by Heimdall and then Vidar. Vidar uses his giant shoe to squash Thrym while Thor retrieves Mjolnir and uses it to obliterate Thrynga. Thor takes the group back to Valhalla to celebrate the return of his hammer but none of the group really feel like celebrating.
“Meetings for the Future”
Magnus, Sam, Alex, Blitz and Hearth are summoned to Helgi's office He informs them that Loki has gone to the Eastern Shores to finish constructing Naglfar and that Sam's mission from Odin is to lead the team to put him back in chains. The next morning, before going to meet Sam, Magnus talks with Alex about him/her teaching Sam to resist Loki's control like he/she can. When he meets Sam, she says that in order to reach the Eastern Shores, they must travel by sea to the old Viking lands of Scandinavia. Sam also says that after this mission is complete, she intends to resign from the Valkyries.
Magnus meets up with Annabeth after managing to get in contact with her through a letter sent by raven. She mentions that New York is crazy right now due to the return of "evil Roman emperors." After telling Annabeth about his mission and making it clear that he has trouble with sea gods, Annabeth agrees to help and says that its time for Magnus to meet Percy.
Cannot be stopped.
Unraveling of the Prophecy
- This is a short and very minor prophecy about Blitzen read by Hearthstone through his runes.
- Randolph Chase stabs Blitzen with the Skofnung sword resulting in a lot of blood.
- Blitzen's friends turn him to stone to prevent dying from the wound.
- Then they use the Skofnung stone to heal him before Thrym's wedding on OSTARA.
- Magnus Chase
- Samirah al-Abbas
- Alex Fierro
- Halfborn Gunderson
- Mallory Keen
- Thomas Jefferson Jr.
- Randolph Chase
- Caroline Chase
- Emma Chase
- Aubrey Chase
- Annabeth Chase
- Amir Fadlan
- Could You Please Stop Killing My Goat?
- Your Standard Rooftop Chase Scene with Talking Swords and Ninjas
- My Friends Protect Me by Telling Me Absolutely Nothing. Thanks, Friends
- A Cheetah Run Me Over
- My Sword Has a Better Social Life Than I Do
- Love Me Some Weasel Soup
- Have You or Someone You Love Ever Suffered From Lindworms?
- I Am Saved From Certain Death By Being Killed
- Never Take a Bubble Bath With a Decapitated God
- The Most Awkward Viking Luau Ever
- What's a Guy Gotta Do to Get a Standing Ovation?
- Samirah and Magnus Sitting in a Tree, T-A-L-K-I-N-G
- Relax, It's Just a Little Death Prophecy
- Cry Me a Blood River. Wait. Actually, Don't
- All in Favour of Slaughterng Magnus, Please Say Aye
- Hearthstone Unleashes His Inner Bovine
- Uncle Randolph Gets on My Naughty List BIG-TIME
- I Need to Learn Many, Many More Cusswords in Sign Language
- Should I Be Nervous that the Pilot is Praying?
- In Case of Demonic Possession, Please Follow Illuminated Signs to the Nearest Exit
- Loiterers Will Be Shot, Then Arrested and Shot Again
- Pretty Sure Hearthstone’s Dad Is a Cow-Abducting Alien
- Yep, His Other Car Is Definitely a UFO
- Oh, You Wanted to Breathe? That’ll Be an Extra Three Gold
- Hearthstone? More Like Hearthrob. Am I Right?
- We Nuke All the Fish
- Let Me Go Immediately, or I Will Make You a Billionaire
- And If You Order Now, You Also Get This Cursed Ring!
- Nøkk, Nøkk
- Somewhere Over the Rainbow, There’s Some Messed-Up Stuff Going On
- Heimdall Takes a Selfie with Literally Everyone
- Godzilla Sends Me an Important Message
- Falafel Break? Yes, Thank You
- We Visit My Favorite Mausoleum
- We Have a Tiny Problem
- Solving Problems with Extreme Fashion
- Meat S’mores Roasting on an Open Fire
- You Will Never, Ever Guess Blitzen’s Password
- Elvis Has Left the Bowling Bag
- Little Billy Totally Deserved It
- When in Doubt, Turn Into a Biting Insect
- Or You Could Just Glow a Lot. That Works, Too
- You Keep Using the Word Help. I Do Not Think It Means What You Think It Means
- We Are Honored with Runes and Coupons
- Pigtails Have Never Looked So Frightening
- Here Comes the Bride and/or the Assassin
- I Prepare for Funkytown Combat
- All Aboard the Cheesy Gordita Express
- A Little Refreshing Poison in Your Face, Sir?
- Hello, Paranoia, My Old Friend
- My Uncle Gets Some Backup Singers
- It’s Hammer Time! (Someone Had to Say It)
- Squirrels in the Window May Be Larger Than They Appear
- Daisies in the Shape of an Elf
- Let’s Try This Whole “Meeting for Coffee” Thing Again
- I Call In Some Favors
- The "Hammer of Thor" refers to Mjolnir, which was lost during The Sword of Summer.
- The name of chapter 49 is "Thrym!". Since Thrym resembles Norm Peterson from Cheers, this would be a reference to the recurring joke on the show where whenever the character entered the bar everyone present would yell out his name in greeting.
- This book won the Stonewall Award, a literary prize for positive representation of LGBT characters.
- On page 135, Riordan makes a typo, having Catherine in place of Caroline.